Solid state circuits that interface with wired communications transmission media typically require protection from transient energy appearing at the interface. This protection typically involves voltage clamping and current limiting elements. Voltage clamps that track the power supply voltage of the interface circuitry are often required. One of the challenges of using this type of protection is that some of the transient energy may find its way into the power supply distribution and cause collateral damage. Transient voltage clamping requirements become more critical for circuits with low impedance interfaces since series current limiting resistors have a detrimental affect on the interface performance. In some cases, protection at a voltage level that is below the level of supply voltage(s) and that tracks any change in voltage may provide improved protection and permit the selection of lower series resistance between the clamp and the protected circuit. Protection that tracks supply voltage changes is especially important when the supply voltage is not regulated as is the case in many telecommunications applications utilizing battery reserve.
Two methods are typically employed to clamp transient voltages to a level that tracks the protected circuit power supply voltage. The simplest method is to use diodes between the line interface and the power supply that are forward biased when transients exceed the circuit supply voltage by at least one diode drop. A problem with this solution is that transient currents are diverted to the supply voltage rail. Power supplies are typically only designed to source current, and transient energy conducted to the power supply rail must be dissipated in the circuitry powered by the supply. A second method is to use a triggered thyristor device that diverts current to ground when transient voltages exceed the supply voltage by a few diode drops. Currents required to trigger this type of device are conducted to the power supply rail. This type of a device results in much lower currents at the power supply interface, but these currents can still be significant when the power supply is lightly loaded.
With these protection methods, transient voltages appearing at the line interface will always exceed the supply voltage. Resistors are usually required between the protection device and the protected circuit to limit transient currents in the interface circuitry. The value of these resistors is constrained by the protection voltage and can have a detrimental affect on the circuit performance.
This type of protection is least effective when transient energy finds its way to the supply rails through multiple interfaces or when the transient is repetitive as is the case with an AC power contact. Currents in the protection diodes or in trigger circuits of protective clamp devices can cause the supply voltage to be pumped up, especially when the power supply is lightly loaded. Some transient events, such as an AC power fault, can result in repetitive transient currents that must be absorbed by the power supply. If the cumulative transient current absorbed by the supply exceeds the load, the supply voltage and tracking protection clamp voltage both are increased and the protected load and possibly other circuitry powered by the supply can be damaged as a result.
Transient or foreign currents find their way into the power supply circuits through multiple paths when the protection clamping voltage exceeds the supply voltage. One of these paths can be the integrated circuit that the protection circuitry is designed to protect. When currents flow through IC interfaces to the supply voltage, causing the voltage at these interfaces to exceed the supply, latch up or destructive conditions may result. If transient voltages are clamped to levels below the supply these latch up conditions can be avoided.
What is required is an alternative protection circuit that will divert transient energy away from the interface circuit and its power source.